The pktgen.txt documentation still claimed that adding same device to
multiple threads were not supported, but it have been since 2008 via
commit e6fce5b9 ("pktgen: multiqueue etc.").

Document this and describe the naming scheme dev@X, as the procfile name
still need to be unique.

Fixes: e6fce5b9 ("pktgen: multiqueue etc.")
Signed-off-by: NJesper Dangaard Brouer <brouer@redhat.com>
Acked-by: NAlexei Starovoitov <ast@plumgrid.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>